1. Field of the Invention
The present invention relates to coating formulations comprising surface-modified silicon dioxides and having improved performance properties and to the use thereof.
2. Description of the Related Art
Coating formulations comprising thixotropic alkyd resins became increasingly of interest at the end of the 1940s. The development began with alkyd resins of standard construction being given an unusually high viscosity, thereby producing a certain thixotropy. A key advance was achieved by reacting oils or alkyd resins with polyamides which are prepared by condensing polymerized linolenic acid with polyamines. The resulting resins display a strongly pronounced thixotropy, so that coating formulations comprising these resins do not run on vertical surfaces—thus concerns about the sagging referred to as “curtaining” are minimized—and also do not drip from the brush. The thixotropy also prevents the settling of pigments of relatively high specific gravity. These types of formulations are therefore suitable for producing household enamels which can be used in particular by laypersons.
Thixotropic alkyd resins can be mixed with the majority of long-oil standard alkyd resins, which allows the degree of thixotropy to be adjusted. This approach is used very frequently, for example, in the production of architectural paints, where a high thixotropy would lessen the flow and gloss of the paints. Thixotropic resins can additionally be used to produce paints featuring eggshell gloss.
In order to increase the consistency of a paint or coating material, there is a need for thickeners, along with suitable binders, solvent mixtures and, where appropriate, pigment/filler fractions. In solventborne coating systems a distinction is made between thickeners and thixotropic agents. The action of thickeners/thixotropic agents is based on different effects such as swelling, gelling, association of micelles, solvation, development of network structures and/or hydrogen bonds and the interaction of these effects. In aqueous systems the degree of thickening is determined by the molecular structure and the molar weight of the hydrocolloids.
One specific group of thickeners are the associative thickeners. They differ from the modified natural substances and the fully synthetic organic thickeners in that, in addition to water-solubilizing hydrophilic groups, their molecular structure also includes hydrophobic end groups or side groups. This gives associative thickeners a surfactant character and the capacity to form micelles. In contrast to the thickeners which act solely by swelling in the aqueous phase, associative thickeners enter into interaction with the latex particles of the binder dispersion and connect them via “micelle bridges.” A notable feature is that associative thickeners thicken fine dispersions, owing to their larger overall surface area, to a greater extent than coarse dispersions.
By means of associative thickeners the rheological behavior of paint systems can be adjusted so that, on the one hand, in a state of low shear the paint has a high viscosity, and so, when the paint is applied to a vertical surface, there is no paint running (curtaining). On the other hand, the mechanical stress in a state of high shear lowers the viscosity (shear-thinning), and so the paint becomes highly mobile, and can easily be conveyed through a narrow nozzle such as, for example, when being applied by spray gun.
Paints and stains, in addition to the abovementioned components, often also include matting agents prepared synthetically or composed of natural materials, which are in the form of fine particles and are used in various applications such as, for example, industrial coatings, coatings on leather and on plastic, and printing inks. The matting agents used in the paints and stains ideally exhibit the following properties: high pore volume, narrow particle distribution, suitable particle size tailored to the respective application, and narrow pore distribution. Silicas (precipitated silicas and silicas prepared on the basis of pyrogenic (fumed) silicas) and/or silica gels possess most of the aforementioned properties, and therefore, are commonly used as matting agents.
When the abovementioned matting agents are used in thixotropic decorating/alkyd paints or in aqueous dispersion-based paints and stains, they often adversely affect the action of the associative thickeners and hence drastically lower the viscosity of the paint.